Exposure to microgravity evokes complex physiological responses that contribute to muscular deconditioning and orthostatic intolerance. The effects of an oral branched chain amino acid solution on muscle protein turnover and fluid balance will be determined during 15-day head-down bedrest. Infusions of ring-13C-phenylalanine and muscle biopsies will determine muscle protein dynamics, while oral 15N-glycine will be utilized to determine whole-body protein turnover. Other body fluid and endocrine responses will also be measured.